It’s not a full screen but a preview mode. Just like on desktop you cannot edit in the preview.
I remember that Don once created a full screen button for mobiles which helps because it hides the title and category field - just like the preview. But I haven’t tried it, so I don’t know if it’s still working.
An alternative in your case could be to hide the gif icon on mobiles or move it to the toolbar instead of the separate button at the bottom. Then upload and preview would share a line which could result in one more line for the composer.